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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="preprocessor/conditional.html" title="c/preprocessor/conditional"><span class="t-lines"><span>#if</span><span>#ifdef</span><span>#ifndef</span><span>#else</span><span>#elif</span><span>#endif</span></span></a></div></div></td></tr>
<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="preprocessor/replace.html" title="c/preprocessor/replace"><span class="t-lines"><span>#define</span><span>#undef</span><span>#,## 运算符</span></span></a></div></div></td></tr>
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<tr class="t-nv"><td colspan="5"><div class="t-nv-ln-table"><div><a href="preprocessor/line.html" title="c/preprocessor/line"><span class="t-lines"><span>#line</span></span></a></div></div></td></tr>
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<p>预处理器于<a href="language/translation_phases.html" title="c/language/translation phases">翻译阶段 4</a> 执行，在编译之前。预处理的结果是将被传递给实际编译器的单个文件.
</p>
<h3><span class="mw-headline" id=".E6.8C.87.E4.BB.A4">指令</span></h3>
<p>预处理指令控制预处理器的行为。每个指令占据一行，且拥有下列格式：
</p>
<ul><li> <code>#</code> 字符
</li><li> 预处理指令（<code>define</code>、<code>undef</code>、<code>include</code>、<code>if</code>、<code>ifdef</code>、<code>ifndef</code>、<code>else</code>、<code>elif</code><span class="t-rev-inl t-since-c23"><span>、<code>elifdef</code>、<code>elifndef</code></span> <span><span class="t-mark-rev t-since-c23">(C23 起)</span></span></span>、<code>endif</code>、<code>line</code>、<code>error</code>、<code>pragma</code> 之一）<sup id="cite_ref-1" class="reference"><a href="preprocessor.html#cite_note-1">[1]</a></sup>
</li><li> 实参（取决于指令）
</li><li> 换行符
</li></ul>
<p>允许空指令（跟随换行符的 <code>#</code> ），而它无效果。
</p>
<h3><span class="mw-headline" id=".E8.83.BD.E5.8A.9B">能力</span></h3>
<p>预处理器拥有源文件翻译能力：
</p>
<ul><li> <b><a href="preprocessor/conditional.html" title="c/preprocessor/conditional">有条件</a></b>编译源文件的某些部分（由 <code>#if</code>、<code>#ifdef</code>、<code>#ifndef</code>、<code>#else</code>、<code>#elif</code><span class="t-rev-inl t-since-c23"><span>、<code>elifdef</code>、<code>elifndef</code></span> <span><span class="t-mark-rev t-since-c23">(C23 起)</span></span></span> 和 <code>#endif</code> 指令控制）。
</li><li> <b><a href="preprocessor/replace.html" title="c/preprocessor/replace">替换</a></b>文本宏，可以连接或加引标识符（以指令 <code>#define</code> 和 <code>#undef</code> ，运算符 <code>#</code> 和 <code>##</code> 控制）。
</li><li> <b><a href="preprocessor/include.html" title="c/preprocessor/include">包含</a></b>其他文件（以指令 <code>#include</code> 控制）。
</li><li> 导致<b><a href="preprocessor/error.html" title="c/preprocessor/error">错误</a></b>（以指令 <code>#error</code> 控制）。
</li></ul>
<p>能控制预处理器的下列方面：
</p>
<ul><li> <b><a href="preprocessor/impl.html" title="c/preprocessor/impl">实现定义</a></b>行为（以指令 <code>#pragma</code> <span class="t-rev-inl t-since-c99"><span>及运算符 <code>_Pragma</code> </span> <span><span class="t-mark-rev t-since-c99">(C99 起)</span></span></span>控制）。
</li><li> <b><a href="preprocessor/line.html" title="c/preprocessor/line">文件名与行信息</a></b>，可用于预处理器（以指令 <code>#line</code> 控制）。
</li></ul>
<h3><span class="mw-headline" id=".E8.84.9A.E6.B3.A8">脚注</span></h3>
<ol class="references">
<li id="cite_note-1"><span class="mw-cite-backlink"><a href="preprocessor.html#cite_ref-1">↑</a></span> <span class="reference-text">这些指令是标准定义的。标准不定义其他指令的行为：它们可以被忽略、拥有一些有用的含义或导致编译时错误。即使忽略，也会在预处理器完成工作时将它们从源码中移除。一种常用的非标准扩展是 <code>#warning</code> 指令，它在编译期间放出一条用户定义的消息。</span>
</li>
</ol>
<h3><span class="mw-headline" id=".E5.BC.95.E7.94.A8">引用</span></h3>
<div class="t-ref-std-17">
<ul><li> C17 标准（ISO/IEC 9899:2018）：
</li></ul>
<dl><dd><ul><li> 6.10 Preprocessing directives (p: 117-129)
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-11">
<ul><li> C11 标准（ISO/IEC 9899:2011）：
</li></ul>
<dl><dd><ul><li> 6.10 Preprocessing directives (p: 160-178)
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c99">
<ul><li> C99 标准（ISO/IEC 9899:1999）：
</li></ul>
<dl><dd><ul><li> 6.10 Preprocessing directives (p: 145-162)
</li></ul>
</dd></dl>
</div>
<div class="t-ref-std-c89">
<ul><li> C89/C90 标准（ISO/IEC 9899:1990）： 
</li></ul>
<dl><dd><ul><li> 3.8 PREPROCESSING DIRECTIVES 
</li></ul>
</dd></dl>
</div>
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
<table class="t-dsc-begin">

</table>

<!-- 
NewPP limit report
Preprocessor visited node count: 1448/1000000
Preprocessor generated node count: 4816/1000000
Post‐expand include size: 33006/2097152 bytes
Template argument size: 4125/2097152 bytes
Highest expansion depth: 13/40
Expensive parser function count: 0/100
-->

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